The purification and recycling of cooling water in light water reactors requires efficient removal of traces of silica from this water in order to prevent silica deposition on the heat exchange tubes of the reactor. However, the problem of silica removal is greatly complicated by the presence of a large excess of borate (typically about 100 to 10,000 times more borate than silica) introduced to control the neutron flux. In such reactors a very high degree of uniformity in operating conditions is required. If borate were to be removed along with the silica, this would cause great expense and technical difficulty because of the necessity to add fresh borate to the recycled water. Also, the absorption and filtration medium would be exhausted at a rate larger by about three orders of magnitude than if silica alone were to be removed. The desired purification system should therefore be capable of efficiently separating silicate from borate and retaining the silica on the purifying medium while avoiding depletion of the borate concentration in the recycled water. Separation of silica from ions which may be present in the water but, unlike silica, do not carry the risk of solid precipitation (e.g., Li.sup.+, Cl.sup.-, NO.sub.3.sup.-) is also desirable.
The term silica as used herein includes the various forms of silica that may be present in any aqueous solution including Si(OH).sub.4, silicates, and colloidal silica.
Organic, synthetic resin ion-exchange columns are used for the vast majority of ion-exchange separations in modern chemical operations. Some separations are relatively simple and may even be quantitative. However, the problem of picking up silica on an ion-exchange resin, even in the absence of interfering ions, is not simple. In routine ion-exchange separations, silicate is preferably left in the effluent, rather than deposited in the ion-exchange column.
Even supposing that silica could be efficiently and cleanly separated on ion-exchange resins, its separation from an overwhelming excess of borate represents a problem to which no solution based on the use of resins has yet been found. The use of strongly basic anion exchangers necessary for removing the extremely weak silicic acid would also take up other weak acids such as carbonic and boric acids. Silicic acid is the weakest of these three acids, with a dissociation constant of 2.1.times.10.sup.-10, slightly below that of boric acid (7.3.times.10.sup.-10).
The situation is further complicated by the fact that boric acid, despite its low dissociation constant value, tends to be absorbed very firmly even by certain weakly basic resins. The greater tendency of borate to adhere to a basic resin as compared with silicate is illustrated by the observation that the gluconate form of a strongly basic resin can be used to retain borate while silicic and hydrocyanic acids pass into the effluent.
Cation exchangers, which are effective in removing cations from borate solutions in the presence of silicate, leave the silica in the effluent together with the borate, and even mixed-bed columns, consisting of a mixture of a strongly acidic cation exchanger and a weakly basic anion exchanger, retain the metal cations but allow other weak acids to exit with the boric acid in the effluent. Soluble silicate and even colloidal silica appear in the effluent, although some precipitated silica accumulates on the ion-exchange resin. Sequential arrangements of cation and anion-exchange columns likewise remove only metal cations and anions of strong acids from boric acid solutions.
It is generally recognized in the chemical literature that inorganic ion exchange materials, especially those based on alumina and silica, are inferior with regard to versatility, stability and selectivity as compared with organic ion exchange resins, which can be "tailor-made" for specific separations and are very versatile and extremely stable under a wide variety of conditions. While a few separations of specific cations, such as Cu.sup.++ and Li.sup.+, have been accomplished by means of inorganic ion exchanges, their main use has been limited to non-specific de-ionization, for instance, in softening and desalination of water, rather than in analytical separations. Even in non-specific applications they have been largely displaced by organic resins. In order to perform specific separations the approach generally accepted at present is to choose among the "tailor-made" organic exchange resins and to optimize the conditions of their use rather than to attempt using inorganic media based on silicates and aluminates.
Highly-selective, "tailor-made" organic resins would appear to be even more preferable in attempting the separation and removal of an ion present at very small concentrations, e.g., silicate, from a solution containing a large excess of another ion, e.g., borate, which is comparable to the former ion in terms of acid-base properties.
It is therefore unexpected that inorganic ion exchange media would prove more useful than specific organic resins under the stringent requirements for high selectivity that exist in a high concentration borate-low concentration silica system, e.g., as in the pressurized water reactor coolant.
Various modifications of alumina have been shown in the art to be capable of picking up ions from solution through a mechanism of ion exchange, absorption, physical adsorption on the surface, or a combination of any of the above. For convenience these mechanisms will be referred to herein as adsorption, without intending to exclude any other reaction mechanisms. The same is true of other hydrous oxides, such as those of iron, thorium, manganese, zinc and magnesium. Because of its amphoteric character, alumina washed with an alkali solution can function as a cation exchanger, while in acidic solution alumina functions as an anion exchanger. The large dependence on pH has been ascribed to the equilibrium equation: EQU Al(OH).sub.2.sup.+ +OH.sup.- .revreaction.Al(OH).sub.3 .revreaction.AlO(OH).sub.2.sup.- +H.sup.+
As a cation exchanger, alumina has been shown to adsorb Li.sup.+ from a solution containing other cations at a carefully controlled pH of 12.6. As an anion exchanger, the properties of alumina are sensitively dependent on pH, surface area and structure.
Anions such as fluoride, oxalate and sulfate can be removed from solutions in preference to phosphate, perchlorate, chloride and nitrate, and they release hydroxide anions from alumina and similar hydrous oxides leading to an increase in pH.
The limitations generally recognized in adsorption and separation of ions by means of surface-active hydrous alumina include the following problems:
a. As a result of the combination of ion exchange and surface adsorption on alumina and similar hydrous oxides, selectivity is usually low and complete elution is difficult.
b. To achieve selective separations, it is necessary to make use of the amphoteric nature of the alumina by adjusting the pH to various values during operation. This requires the addition of relatively large volumes of electrolytes.
c. Ion exchange is most efficient at very high (12-13) or very low (-1 to +1) values of pH. However, alumina is soluble to a considerable extent in strongly acidic and strongly basic solutions, and the effluent can be expected to be contaminated with significant levels of dissolved aluminum.
d. The capacity of alumina, as measured at a pH of more than one, is about 0.17 meq/ml of column, which is much less than the capacity of an organic anion-exchange resin.
e. Activated alumina cannot be used for some separations because certain solute types undergo chemical reactions such as oxidation, complex formation and polymerization at the reactive sites.
f. The limitation which appears to be most serious in attempting the separation and removal of silicate from boric acid solutions is that the strongly basic sites of alumina show a preferential adsorption of acidic species according to their acid strength. Strong acids are most strongly bound, while the weaker acids can be separated in order of their pk.sub.a values when basic (or, to a lesser extent, neutral) eluents are used. Since boric acid (k.sub.a =7.3.times.10.sup.-10) is slightly stronger than silicic acid (k.sub.a =2.1.times.10.sup.-10), silicate is not expected at first sight to adhere preferentially to alumina in the presence of borate.
Alumina, as well as other metal oxides, especially those of iron and magnesium, and metal powders, such as iron and aluminum, have been used to remove silica from water, especially from boiler feed water, to prevent the formation of scales, although in general organic ion exchange resins are preferred at present in reducing silica concentrations.
The effectiveness of alumina in adsorbing silica depends strongly on pH. A basic pH is favored for preparation of the alumina as well as during the adsorption process itself, where optimum removal is obtained around pH 8. The pH is maintained below about 9 (and above about 5) to prevent introduction of dissolved ionic aluminum. The addition of salts such as MgSO.sub.4 and Fe.sub.2 (SO.sub.4).sub.3 has been known to improve the results, and NaCl is particular is known to enhance silica removal from the liquid by forming colloidal aluminosilicate flocculant. However, flocculation techniques are usually inefficient and the introduction of added salts requires their removal by other means such as cation exchange resins.
The use of various aluminas has been reported to result in reductions of silica levels in water, e.g., from 68 to 5 ppm (Behrman et al, Ind. Eng. Chem. 32, 468 (1940)-), from 95.2 to 2.8 ppm (Lindsay et al, Ind. Eng. Chem. 31, 859 (1939)), and from 140 to about 70 ppm and in the presence of NaCl to about 20 ppm (Wey et al, Colloq. Intern., Centre Natl. Rech. Sci. (Paris) 105, 11 (1962)). Wohlberg et al, Los Alamos Report, LA-5301-MS (1973) indicates that silica levels in tap water of approximately pH 8 can be reduced from about 82 ppm to below one ppm employing a column of 80-20 mesh adsorbent alumina. These investigators also reported treating a higher silica concentration solution from a cooling tower employing stirred alumina in a beaker and obtaining a reduction in silica content from 146 ppm to only 83 ppm. The efficiency of the process depends not only on the pH of adsorption (see above) and the pH of previous treatment of the alumina (basic alumina reduces silica levels from 82 to 1.8 ppm, acidic alumina to 0.8 ppm, neutral alumina to one ppm), but also on grain size and structure. Granular activated alumina (Behrman et al), dried, hardened gel (Liebknecht U.S. Pat. No. 1,860,781), and freshly precipitated alumina (Lindsay et al), have been specified for use in silica removal.
In summary, survey of the literature shows that alumina and inorganic hydrous oxides are generally considered less effective and less useful than organic ion exchange resins in separatory processes. Most seriously, the specificity of alumina in removing silica from solutions containing an excess of other ions has not been identified in the prior art.
Moreover, according to the literature, e.g., Perry et al, Practical Liquid Chromatography, Plenum Press, New York (1972), pages 62-64, alumina is not expected to separate and remove silica from solutions of anions of stronger or comparably strong acids, such as borate ion, and is expected to be inferior in selectivity as well as in stability, versatility and capacity in comparison with organic ion-exchange resins.
The use of alumina to remove boron, i.e., borates, from solution is shown in Gustafson U.S. Pat. No. 2,402,959. This patent is not concerned with treating solutions containing silica as well as borates.
In the specification and claims unless otherwise indicated, when reference is made to the amount of silica, it is expressed as ppm (parts per million) calculated as silicon and, when reference is made to the amount of borate, it is expressed as ppm calculated as boron. Unless otherwise indicated, all parts and percentages are by weight.